The electrochemical behavior of 6-MP was studied by cyclic voltammetry at a hanging copper amalgam dropping electrode (HCADE). It was found that 6-MP could form a complex with the Cu(II) stripped from the HCADE, showing a new peak at -0.19V in the medium of 0.1mol/L LiClO4-0.5mol/L HClO4 solution. The mechanism of the reaction was proposed. This new peak was sensitive and could be used for the determination of trace 6-MP by differential pulse adsorption cathodic stripping voltammetry (DPAdCSV). The linear range was from 3.6 x 10^-10 to 5.3 x 10^6 mol/L, and the detection limit was about 1.2 x 10^-10 mol/L (S/N=3). The method was also successfully applied to the determination of 6-MP in pharmaceutical tablets.